With the discovery of ferrocene in 1951, the cyclopentadienyl ligand (Cp) gained prominent renown in the field of organometallic chemistry of transition metals and advanced rapidly to dominate in form of bis(cyclopentadienyl) or metallocene complexes the metals of the d-block and p-block. With the development of tris-(cyclopentadienide) of the lanthanoids and of group 3 by WILKINSON and BIRMINGHAM, this ligand ultimately also achieved a successful breakthrough in the field of organometallic chemistry of rare earth metals (RE) that is highly sensitive to hydrolysis; syntheses of different bis-(cyclopentadienyl) derivatives followed thereafter.
The term metallocene, as it became known due to ferrocene, is used not only for pure bis-(cyclopentadienyl) complexes but also for complexes with metals that carry further anionic ligands (X) or neutral ligands (L) in form of CpR2RE(X)n(L)m (FIG. 1). CpR2RE(+II) and CpR2RE(+III) halogenides are excellent starting compounds for alkyl, hydrido or amido species. These in turn possess, ultimately, a reactive function for catalytic applications, primarily in hydrometalation reactions, cyclization reactions and polymerization reactions of olefins. Electronic or steric parameters can be varied by organyl substituents R on the cyclopentadienyl unit. Especially prominent variants of C5H5 are pentamethylcyclopentadienyl C5Me4, indenyl and fluorenyl units (in general, CpR).
A bridging Y (in most cases, SiR2, —CnH2n—) of both CpR units results in ansa-metallocenes. The conformation of the complex becomes fixed, whereby the LEWIS acidity of the metal center is successfully increased, because the catalyst center is rendered more accessible for larger substrate molecules in the catalysis. Correspondingly, the activity of the ansa-lanthanocene catalyst {Me2Si(C5Me4)2RE(H)}2 is ten times higher in comparison to the unbridged lanthanocene {Cp2RE(H)}2 in the ethylene polymerization as well as in the copolymerization of ethane with 1-hexene.
Achiral ansa-metallocenes of the Me2Si(C5Me4)2LnR and Me2Si(C5Me4)(C5H4)LnR types and chiral ansa-metallocenes were prepared in a broad variety. Chiral ansa-lanthanocenes play are crucially important in asymmetrical hydroamination, as described for the first time by MARKS shortly after the development of the highly efficient intramolecular hydroamination by achiral lanthanocenes.
The successful use of ansa-metallocenes in polymerization catalysis has intensified the interest in further reducing the steric requirements and in studying pure half-sandwich complexes with one or two reactive functions CpREII(X) or CpREIII(X)2, respectively. Since the literature contains relatively little information regarding mono-Cp- as opposed to bis-Cp-systems, it is already obvious that the synthesis of such species is associated with difficulties. In particular, when a sterically undemanding Cp-moiety is to be inserted, in the ligand substitution, there ensues time and time again the formation of thermodynamically favored metallocene or tris-Cp complexes.
In addition, the more electropositive nature of the RE central atom results in a stronger ionic bond, which means the compound tends to undergo ligand substitution reactions (ligand scrambling). Although most neutral half-sandwich complexes do not show any activity in the polymerization of olefins, a transfer, however, by way of, for example, MAO or boranes, such as [B(C6F5)3], or borates, such as [Ph3C][B(C6F5)4] or [Ph2MeNH][B(C6Fs)4], respectively, to a cationic active species resulted multiple times in the successful use in the context of regio- and stereospecific polymerizations of olefins, or also in copolymerization reactions with cyclic and aromatic monomers.
A more successful improvement resulted from the substitution of a cyclopentadienyl moiety in an ansa-metallocene with another anionic or neutral donor D obtaining the so-called constrained geometry complexes. Contrary to Cp-rings, which, by way of being inert, strongly protective ligands (spectator ligands), serve for the complex formation of a reactive metal center, the donor D has a more faceted function: it can be hemilabile during catalysis and be subject to temporary decomplexation without degradation of the catalyst system, or it can flexibly contribute 2, 4 or 6 electrons in α- and π-bonds to the central atom.
BERCAW presented the first constrained geometry complex with the Cp-Y-D-ligand motif in 1990 (P. J. Shapiro, E. Bunel, W. Schaefer, J. E. Bercaw, Organometallics 1990, 9, 867-869) in form of a dimeric Sc-hydrido complex with the chelate unit CpSiN (FIG. 2).
Aside from the many prominent group 4 compounds, ansa-cyclopentadienyl complexes with rare earth metals form a class of interesting homogeneous catalysts that are also used on an industrial scale (EP 0416815A2).
Some novel structural motifs for constrained geometry complexes have been developed previously by Sundermeyer et al. They contain a chelating iminophosphorane unit with a strongly basic nitrogen atom (FIG. 3). Initially, based on the isoelectronic relationship of the dianionic unit CpSiN of the classic CGC relative to the monoanionic unit CpPN, a new series of ligands of different steric requirements and electronic donor nature was prepared. The first lutetium complex of the new system was published in 2005. I was possible to study a series of CpPN complexes of further rare earth metals and metals of the groups 3 and 4, where the CpPN unit functions as a chelate ligand, as well as the activity thereof in intramolecular hydroamination and in olefin polymerization (WO2009068000). Moreover, experiments were conducted regarding the synthesis of group 4, aluminum, chromium, vanadium and further rare earth metal CG complexes. Independently but simultaneously in relation to the above, BOURISSOU et al. published DFT calculations and experimental works regarding fluorenyl-based FluPN ligand systems and the complex formation thereof on group 4 metals.
As mentioned previously, in CpPN complexes, the phosphacene unit is bonded directly to the C5-ring via the phosphor atom and is therefore a component of the structurally verifiable chelate unit. The monoanionic CpSiNP ligand is a hybrid of the novel monoanionic CpPN and the classic dianionic CpSiN ligands, where the phosphacene unit is exocyclically arranged relative to C5-Si—N chelate ring that is the identified target. Also related to the ligand system that was described for the first time in 2010 are such compounds with Me2Si-bridged cyclopentadienyl-imidazolin-2-imine ligands the N-atoms of which are also a component of an extremely basic, ampholyte-ionically constructed neutral ligand, namely of an imidazole ring. The work group TAMM published constrained geometry complexes with this ligand system for chromium (S. Randoll, P. G. Jones, M. Tamm, Organometallics 2008, 27, 3232-3239), for rare earth metals and alkaline earth metals (T. K. Panda, C. G. Hrib, P. G. Jones, J. Jenter, P. W. Roesky, M. Tamm, Eur. J. Inorg. Chem. 2008, 4270-4279). Also to be noted in the present context is a further, however, dianionic, CpPN ligand system of the general nomenclature (CpRPRNR)2− with a phosphor(III) atom in the bridge by the work group SUNDERMEYER, because the corresponding titanium complexes produce linear, highly molecular polyethylene of high activity (J. Sundermeyer et al. Eur. J. Inorg. Chem. 2002, 678-691; J. Organomet. Chem. 2001, 640, 21-28).
Further to be noted as known in the art are monoanionic, bidendate phosphoniumdiylide ligands (“CPC,” Type A, FIG. 4). Complexes of this flexible chelate-ligand system with metals of the total periodic table have been researched (H. Schmidbaur, Angew. Chem. Int. Ed. 1983, 22, 907-927). To be mentioned in this field of interest is primarily the first homoleptic, dissolvent-free organyl complexes of lanthanoids by SCHUMANN. This compound class [(CPC)3RE] with KZ=6 is sufficiently stabilized by ligands with two α-donors of Type A (FIG. 4) for lutetium and, ranging to the largest of the lanthanoids, lanthanum (H. Schumann, F.-W. Reier, J. Organomet. Chem. 1982, 235, 287-294). This is why their characterization and their reactivity were studied. Aside from homoleptic complexes, there are also heteroleptic complexes of rare earths of Type A, which are in most cases stabilized by Cp-co-ligands, and the reactivities of which were studied.
However, not only the classic phosphonium diylides have awakened scientific interest, calling for attention are also a number of phosphor-bridged ansa-metallocene ligands of Type C (CpPCp) with two π-donors in the chelate unit. Complexes of alkali and alkaline earth metals, group 4 metals and iron were prepared with such bis-cyclopentadienyl-phosphonium diylide ligands of Type C. However, not much is known regarding their reactivity. In one example, a CpPCp-iron complex itself serves as catalyst in the polymerization catalysis (H. Schumann, S. Hohmann, Chemiker-Zeitung 1976, 100 (7-8), 336).
Although cyclopentadienylidene phosphorane of type R3P═C5R4, the structural motif of which is underlying the new ligand class of type B, has been known for a long time as an ampholyte-ionic ligand in the literature, reporting on the advantageous use of complexes containing deprotonated forms of this ligand has been lacking to date (F. Ramirez, S. Levy, J. Org. Chem. 1956, 29, 1333).